if you haven't completed the final stage on the achievement for buying tools, coins are handy for it. Don't go through the "shop" function (since it only lists the tools you can buy with diamonds). Instead, simply scroll down the tools list in the corner to the flashlights, and you can buy them 1 at a time for coins. I had plenty of coins, so I just bought flashlights until I had earned all stages. (Didn't take too long, and the final chest had good stuff in it).